1. Field of the Invention
The present invention relates to masks (or reticles) and other devices for use in microlithographic techniques, especially for use in forming contact holes in semiconductor products with improved depth of focus. The invention may be used to form isolated contact holes, arrays of contact holes and other structures. The present invention also relates to a method of determining a desired three-tone pattern for a microlithographic mask. The method, which may be performed on one or more programmed microprocessors, may involve the selection of one set of dimension data out of a plurality of dimension data sets, where the dimension data sets correspond to different mask patterns, and the selected set is the one that provides the greatest depth of focus.
2. Description of Related Art
A semiconductor device can be fabricated by photolithography, in which light is transmitted through a patterned mask (or reticle). The pattern on the mask is exposed on a layer of photoresist to form the desired feature or features in the semiconductor device. Examples of such features are isolated contact holes and contact holes formed in closely packed arrays. In certain circumstances, it is desirable to make contact holes with very small critical dimensions. The “critical dimension” is typically the diameter of the hole in the plane of the surface of the semiconductor device. Some devices require contact holes with critical dimensions that are less than the wavelength of the light that is used to expose the photoresist. A dimension that is less than the wavelength of the exposing light is referred to as a “sub-resolution” dimension.
A number of binary and phase-shifting masks have been proposed in the prior art. Such masks are shown, for example, in U.S. Pat. No. 6,114,071 (Chen et al.), U.S. Pat. No. 6,077,633 (Lin et al.), U.S. Pat. No. 6,022,644 (Lin et al.) and U.S. Pat. No. 5,707,765 (Chen). There is still a need in the art, however, for a three-tone mask (or other multi-tone mask) that can form holes and other structures with small critical dimensions and minimum side-lobing and with improved depth of focus. Depth of focus is especially important in connection with the formation of small structures in non-flat surfaces. Where the wafer is not flat, it may be necessary to image a pattern at different distances from the lithography system with essentially the same fidelity. In addition, it may be necessary to allow for wafer positioning in the system, wafer curvature, focal plane curvature, etc.
Moreover, there is a need in the art for an economical method of making three-tone masks (or other multi-tone masks) for use in the formation of small critical dimension features with minimal side-lobing and large depth of focus.